"The current U.N. regular budget calls for $5.4 billion over two years. By a formula based on the size of a country’s economy and other factors, America’s assessment is set at 22 percent, or about $1.2 billion.
Based on the last Obama administration budget, the United States provided $1.2 billion to the U.N. over two years, and Portman is correct.

But that’s just for the U.N.’s regular budget. There are over a dozen other U.N. agencies and operations that American dollars also support. The single largest is peacekeeping with a price tag of about $7.8 billion this year alone. The U.S. share of that cost is over 28 percent, or $2.2 billion.

The next three U.N. agencies -- all roughly in the $100 million per year range -- are the World Health Organization, the Food and Agriculture Organization and the International Atomic Energy Agency.

All told, the United States provides about $3.3 billion a year to finance U.N. activities. A 40 percent cut would save taxpayers about $1.3 billion out of the current $4.1 trillion U.S. federal budget. The impact on the U.N. would be substantial, although the precise effect would depend on how the cuts are spread around."

https://www.politifact.com/global-news/statements/2017/feb/01/rob-portman/us-contribution-un-22-percent/

President Trump is calling for a 40% cut in the USA funding to the United Nations. How refreshing we have a business man in the White House negotiating these deals